What Is Web Mercator? The Map Projection Behind Every Web Map

Almost every interactive map you have ever used is drawn in Web Mercator. Google Maps, OpenStreetMap, Mapbox, MapLibre, Apple Maps, MapAtlas: all the same projection, identified in the EPSG database as EPSG:3857. The reason is simple: it is the projection that scales cleanly into a tile grid, keeps shapes recognisable at any zoom, and works well as a default backdrop for almost every business map use case.

This guide explains what the projection actually is, why it distorts area in the way it does, and when you should use something else.

A Projection in 30 Seconds

The Earth is approximately a sphere. A computer screen is flat. A map projection is a mathematical function that turns positions on the sphere (latitude, longitude) into positions on the flat surface (x, y). Every projection trades off something: shape, area, distance, or direction. There is no perfect projection for all uses, which is why cartography exists as a discipline.

Web Mercator preserves shape and direction. It distorts area. The further you are from the equator, the bigger features look relative to their true size on the ground.

The Math, in One Paragraph

For longitude lng and latitude lat in radians:

• x = R * lng
• y = R * ln(tan(pi/4 + lat/2))

where R is the radius of the spherical Earth used by the projection (6378137 metres in EPSG:3857). The tile grid maps these x,y values into integer pixel coordinates at each zoom level, with zoom 0 covering the world in a single 256x256 tile and each subsequent zoom doubling the resolution.

The latitude bound is set at approximately +/- 85.0511 degrees because the formula goes to infinity at the poles. This is why polar regions are clipped on every Web Mercator world map you have ever seen.

Why Areas Get Stretched

The reason area distortion grows toward the poles is that the y-axis is not linear in latitude. The ln(tan(...)) term makes the spacing between latitude lines increase as you move north or south. At the equator, one degree of latitude takes the same vertical space as one degree of longitude. At Helsinki, one degree of latitude takes nearly twice the vertical space.

Two famous side effects:

• Greenland and Africa. On a Web Mercator world map, Greenland looks roughly the same size as Africa. In reality Africa is about fourteen times larger. The same illusion makes Antarctica look like a continuous strip across the bottom of the world.
• Russia. Russia looks enormous because most of it sits in the high-latitude band where vertical distortion is multiplying its apparent area.

For street-level maps, none of this matters. A neighbourhood in Berlin and a neighbourhood in Sydney both render at almost their true shape. But for thematic maps that compare values across continents, this distortion is a real problem.

EPSG:3857 vs EPSG:4326

You will see both EPSG codes in any geospatial codebase.

• EPSG:4326 (WGS84 lat/lng): angles. The way you store coordinates. The way GPS reports them. The way GeoJSON encodes them. This is the universal coordinate system.
• EPSG:3857 (Web Mercator): a projection. The way coordinates are turned into pixels for display.

In production, work in 4326 everywhere except at the rendering boundary. PostGIS columns: geometry(point, 4326). JSON payloads: [lng, lat] in WGS84. Map library input: 4326 coordinates that the renderer projects internally.

Storing data in 3857 is almost always wrong. The moment you need to compute a distance, query a different map provider, or pipe through a GIS tool, you have to project back to 4326 and the conversion costs you precision.

Why Web Mercator Won the Web

When Google launched Maps in 2005, the team chose Web Mercator because it was the simplest projection that produced square tiles at every zoom level. Square tiles slot cleanly into a power-of-two pyramid: zoom 0 is one tile, zoom 1 is four, zoom 2 is sixteen. Every other major web map provider followed the same convention because tile sets had to be interchangeable. Twenty years later, Web Mercator is the unspoken default. Almost every map style, every routing engine, every analytics overlay assumes 3857 unless told otherwise.

Switching to a different projection on the web means rebuilding the tile pyramid. It is technically possible (Mapbox supports a globe view, deck.gl supports custom projections), and it is becoming more common for global view modes, but for street-level maps Web Mercator is sticky.

When to Use Something Else

Use Web Mercator for almost every interactive city, regional, or country-level map. Switch projections in two scenarios:

• Thematic global maps that compare values across regions. Population density, election results, climate variables, anything where the visual area carries meaning. Use Albers Equal Area for North America, Lambert Conformal for Europe, Mollweide or Equal Earth for the entire planet.
• Polar maps. Near the poles Web Mercator falls apart. Use a polar stereographic projection for the Arctic and Antarctic.

For internal GIS analysis (spatial joins, distance buffers, overlay analysis), work in WGS84 angles or in a local projected system designed for the country in question (UTM zones, ETRS89 for Europe). Reproject only at display time.

How MapAtlas Handles It

MapAtlas serves Web Mercator tiles by default for the Dynamic Maps product, with the same EPSG:3857 schema every modern web map library expects. Coordinates returned from the Geocoding API and Search API are always in WGS84 lat/lng (EPSG:4326), so you store the source-of-truth coordinate and let the renderer handle projection. For a deeper dive into projection tradeoffs and when to switch, see the Map Projections guide.